1. Field of the Invention
The present invention relates to signal processing for image signals and audio signals, and particularly to a signal processing method and system for removing noise from a signal on which an impulse noise has been superimposed.
2. Description of Background
When a variety of information is coded for characters, colors, sounds and images, for example, the coded signals thus obtained, such as those for image and audio signals, have signal values that, unlike sequential analog signals, correspond to coded numerical values at individual discrete points (hereinafter referred to as “signal points”). And these multiple signal points are contiguous, based on a predetermined time base order or a predetermined spatial order. The definition for the contiguous arrangement of signal points in a time based order is that, in the case of audio signals, sounds are expressed by sequentially transmitting, at predetermined time intervals, signal values for signal points that represent separate individual time elements in the time based order. Further, the definition of a contiguous arrangement in the spatial order is that, in the case of image signals, an image is expressed by transmitting, at predetermined spatial intervals, signal values located at signal points that represent spatially separated individual pixels. There is a case wherein, for a variety of reasons, impulse noise is superimposed during the coding or the transfer of a coded signal, such as an audio signal or an image signal. Conventionally, a median filter is generally employed in order to remove impulse noise from a signal. A median filter is one that, instead of providing for a coded signal a signal value at a signal point, whereat noise is superimposed, provides as a signal value a middle (median) value at a signal point within a predetermined range, at the center of which is located the signal point for the signal on which noise is superimposed. By employing this median filter, an isolating point can be removed while the edge of the signal can be maintained.
In order to effectively remove impulse noise, a number of conventional techniques are used for devising a median filter employment method by the prior art. According to one technique in the prior art, in accordance with the state of an input image signal and the state of a surrounding signal, when a signal to be processed includes impulse noise, the output of a median filter is provided in the spatially vertical or transverse direction. Whereas when a signal to be processed does not include impulse noise, the signal to be processed is simply provided.
In a different technique, when impulse noise is present in a flat area of an image signal, the average value of surrounding image signals is provided for a pixel to be processed in order to remove the noise.
Yet in another technique, the medians of signals to be processed and preceding and succeeding signals are extracted and are added at a predetermined ratio to the values of input signals, or to the averages of the input signal values and the preceding and succeeding signal values.
As described above, consequently in order to remove impulse noise from an image signal or an audio signal, generally, a median filter is employed. However, a median filter can not effectively restore a detail signal or a signal wherein short rises and falls are repeated and it is desirous to employ a new technique that takes these requirements into consideration.